Here’s an explanation from the style guide: “As a general rule, do not use the serial/Oxford comma: so write ‘a, b and c’ not ‘a, b, and c’. But when a comma would assist in the meaning of the sentence or helps to resolve ambiguity, it can be used – especially where one of the items in the list is already joined by ‘and’ [for example]: They had a choice between croissants, bacon and eggs, and muesli.”
A video (with NSFW lyrics) uses the Oxford comma as a metaphor for the small stuff one should not sweat.